In-Situ Interferometer Arrangement

ABSTRACT

An in-situ interferometer includes an image modifying optic that produces light ray bundles. The light ray bundles are projected onto a reticle with a plurality of measurement fiducials encoded onto a face of the reticle. The measurement fiducials are exposed onto a sensing plane and their locations measured. Aberrations in the projection system are determined from the measurements of the exposed reticles.

REFERENCE TO PRIORITY DOCUMENT

This is a divisional of U.S. patent application Ser. No. 11/173,748filed on Jul. 1, 2005, which is a divisional of U.S. patent applicationSer. No. 10/623,364 filed on Jul. 18, 2003, now U.S. Pat. No. 6,963,390which claims the benefit of priority of U.S. Provisional PatentApplication No. 60/397,312 filed on Jul. 19, 2002, all of which areincorporated by reference in their entirety herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to processes for semiconductormanufacturing and more particularly to characterizing and monitoringlithographic projection lenses in-situ.

2. Description of the Related Art

Projection imaging systems, also referred to as lithographic projectionsystems, have many uses in various industries. For example, lithographicprojection systems are used to produce patterns on semiconductormaterials for use as integrated circuits and flat panel displays. Theperformance of lithographic projection systems greatly influences themanufacturability and cost of manufacturing semiconductor chips and flatpanel displays.

In general, the performance of lithograph projection systems is limitedbecause of aberrations, which are the deviation of a projection lens'performance from a “perfect” lens or from the diffraction limit. As theresolution required from lithography projection systems increases, forexample as low as 100 nm and below, the ability to measure the state ofthe optical aberration of projection lenses becomes increasinglyimportant. For example, aberrations as small as 10 milliwave (mλ) orless can cause significant shifts and distortions in patterns.

Techniques have been developed to compensate for aberrations inlithography projection systems. For example, distortion and fieldcurvature data from images exposed using a lithography projection systemare used to design “figured” optical surfaces that are placed in theoptical path of the projection system to compensate for aberrations ofprojection systems. A drawback to these techniques is that they onlyconsider distortion and field curvature aberrations. Distortion andfield curvature correspond to the lowest order aberrations of an imagingsystem, namely field dependent tilt and focus. In order to ascertain thedegree of correction and method of correction useful for higher orderaberrations, data in addition to distortion and field curvature areneeded.

Application of a conventional interferometer to a projection imagingsystem can provide high quality wavefront data. However, a drawback touse of a conventional interferometer is that it requires removing, orsignificantly altering or disturbing, the lens column of thelithographic projection system. Removal of the lens column can introduceuncertainties into the measurement and require significant downtime fromproductive operation.

Due, at least in part, to the drawbacks in using a conventionalinterferometer in-situ, techniques have been developed for determiningdistortion, field curvature, best focus, astigmatism, and the aerialimage of a projection imaging system in-situ. Of these various in-situtechniques, the greatest amount of information is usually provided byin-situ aerial image measurements. However, a drawback to in-situ aerialimage measurements is that the light level is generally low, leading tolong exposure times or poor signal to noise ratios. In addition, thereconstruction of the aberrated wavefront is ambiguous unless severalout of focus exposures are performed.

Thus, there is a need for an improved technique to measure the opticalaberrations of lithography projection systems. The present inventionsatisfies this need

SUMMARY

A method and apparatus of in-situ measurement of lens aberrations thatincludes producing an illumination source at low partial coherence withchief rays that vary regularly as a function of position. The modifiedsource illuminates a reticle that includes an optical element and a faceencoded with measurement fiducials. The measurement fiducials areexposed onto a sensing plane. The relative positions of the exposedmeasurement fiducials on the sensing plane are measured. Areconstruction of the aberration is made from the measurements and knownrelative positions of the measurement fiducials of the encoded face.

An illumination source for in-situ measurement of lens aberrations caninclude a light source and a condensing lens configured to accept lightfrom the light source. This is a simple effective source (ES) and isused for illustration. An illumination modifying optic is placed withinthe effective source, with the illumination modifying optic configuredsuch that light from the light source that passes through theillumination modifying optic and condensing lens forms a plurality oflight ray bundles with corresponding chief rays. A lens is between theeffective source and the encoded face, wherein angles of incidence ofthe chief rays within the respective bundles onto the lens varysufficiently to overfill the lens pupil.

A reticle for in-situ measurement of lens aberration determination caninclude an array of field points, wherein each field point comprises anarray of fiducials. The reticle also includes an array of opticalelements associated with each of the field points.

Other features and advantages of the present invention should beapparent from the following description of the preferred embodiment,which illustrates, by way of example, principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a prior art technique of in-situinterferometer measurements.

FIG. 2 is a block diagram illustrating aspects of a projectionlithography tool and embodiments of this invention in a projectionlithography tool.

FIG. 3 is a diagram illustrating the appearance of an effective sourceat a conjugate aperture stop with no illumination modifying opticpresent.

FIG. 4 is a diagram illustrating an example of an illumination modifyingoptic including an opaque disc with a hole in the center.

FIG. 5 is a diagram illustrating a source after an illuminatingmodifying optic has been installed in the effective source.

FIGS. 6A-6G are block diagrams illustrating different examples forfiducials.

FIG. 7 is a diagram illustrating a 5×5 array of measurements fiducialsacross a field point.

FIG. 8 is a diagram illustrating a 5×5 array of measurement fiducialsprojected onto a sensing plane.

FIG. 9 is a diagram illustrating a reference array projected onto asensing plane.

FIG. 10A is a diagram illustrating a fiducial that has interspersed subresolution features.

FIG. 10B is a graph that illustrates examples of intensity profiles thatcan be produced by measurements fiducials.

FIG. 11A is a side view of a reticle with optical elements attached.

FIG. 11B is a plan view of the reticle shown in FIG. 11A

FIG. 11C is a plan view of a reticle with a reference array.

DETAILED DESCRIPTION

Methods of in-situ interferometry are described in U.S. Pat. No.5,828,455 entitled “Apparatus, Method of Measurement, and Method of DataAnalysis for Correction of Optical System” issued Oct. 27, 1998, toSmith et al.; and U.S. Pat. No. 5,978,085 entitled “Apparatus Method ofMeasurement, and Method of Data Analysis for Correction of OpticalSystem” issued Nov. 2, 1999 to Smith et al., both of which areincorporated by reference herein in their entirety.

The two patents referenced above describe an in-situ interferometer thatincludes an image matching optic, an encoded face (EF), and an apertureplate (AP). The image matching optic increases the diversity of rayangles impinging on the encoded face so as to fill an entrance pupil ofthe lithographic projection system imaging objective (IO). The EFprovides structures (typically at the reticle object plane) that areimaged onto a wafer and subsequently positionally measured. The APrestricts the size of the individual ray bundles and thus determines thewave front reconstruction resolution. While this in-situ interferometeris effective, it would be advantageous to reduce its complexity and thenumber of components that are needed.

FIG. 1 illustrates a system that can implement the technique describedin the two Smith patents (U.S. Pat. No. 5,828,455 and U.S. Pat. No.5,978,085). FIG. 1 is a block diagram illustrating a projection system100. FIG. 1 schematically illustrates how information about aberrationsare obtainable. Point P 104 in the reticle plane 103 has an apertureplate AP 108 interposed between it and the first element of the opticalsystem. Of the rays 1, 2, 3, 4, 5 emitted from P, only ray 4 passesthrough the opening O 106 in the aperture plate AP 108, so that ray 4 istransmitted by the IO 102 and projected onto a wafer in image plane 112.Aberrations of the imaging objective 102 cause ray 4 to deviate (drawnas a dashed line) from the path that an unaberrated image objectivewould produce (drawn as a solid line). The aberrations cause the ray tointersect the wafer plane at a transverse position PI 110 differing fromintersecting position for an imaging objective without aberrations (a“perfect” imaging objective) by an amount proportional to grad(Φ(u)). Atthe reticle, nearby point P′ has only a small bundle of rays centered onray 1′ passing through the aperture O 106 in the aperture plate AP 108,through imaging objective 102, and intersecting the image plane 112 atthe point P′I 110′. The deviation of ray 1′ from its ideal imaging pointis also proportional to grad(Φ(u′)), u′ being the angle or position ofray 1′ as it passes through the aperture stop AS (see solid line(unaberrated) and dashed line (aberrated)).

The techniques of the Smith patents include analysis of the wavefront ata plurality of field points over the entire lens train. The techniqueincludes using an aperture plate 108 that includes a plurality ofopenings 106. In FIG. 1, each opening 106 is centered underneath aneighborhood of points 104 that are accepted into the entrance pupil ofthe IO 102. Light passing through the points 104 and through allopenings 106 in the aperture plate 108 will produce points at the imageplane 112 corresponding to the number of openings 106 in the apertureplate 108. The totality of all the arrays of points whose centroids canbe measured and reconstructed yield an aberrated wavefront φ(u; x) at anumber of discrete points across the image plane 112.

While this technique overcomes many of the shortcomings of othertechniques it requires inserting a reticle 103 and an aperture plate 108between the image matching optic 101 and the IO 102, thereby decreasingthe light level. In accordance with the invention, an in-situinterferometer system can be provided without an aperture plate.

FIG. 2 is a block diagram of a lithography projection tool 220. Ingeneral, the lithography projection tool 220 includes an illuminationsource at low partial coherence with chief rays that vary regularly as afunction of position. The modified source illuminates a reticle 208 thatincludes an optical element and a face encoded with measurementfiducials. The measurement fiducials are exposed onto a sensing plane242. The relative positions of the exposed measurement fiducials on thesensing plane are measured. A reconstruction of the aberration is madefrom the measurements and known relative positions of the measurementfiducials of the encoded face.

Illumination Matching Optic

The illumination matching optic (IMO) 202 includes an effective lightsource (ES) 204 and an optical element illustrated as a lens (L) 206that is mounted to a reticle 208. The effective source (ES) 204 includesa light source (LS) 210 and a condensing lens (CL) 212 that focuses thelight onto a lens 206 mounted on the reticle 208.

The effective light source (ES) 204 also includes an illuminationmodifying optic IM 214 that is located at a conjugate aperture stop(CAS) 216 of the lithography projection tool 220. Within the projectionlithography tool 220, the conjugate aperture stop (CAS) 216 is imaged bythe combination of the condenser lens (CL) 212 and other effectivesource optics (not shown) and upper lens group (ULG) 230 to aperturestop (AS) 232.

FIG. 3 is a diagram illustrating the appearance of the effective source(ES) 204 at the conjugate aperture stop (CAS) 216 and aperture stop (AS)232 if the illumination modifying optic (IM) 214 is removed, see FIG. 2.The lens (L) 206 pupil (P) 304 is the image of the aperture stop in theobject space. Because the aperture of the system limits the size of theaxial cone of energy that will pass through the optical system, the lenspupil determines the amount of energy accepted by and emitted from theoptical system. The maximum cone angle of light accepted or emitted byan optical system is expressed as its numerical aperture. The hatchedregion 302 illustrates the effective source (ES) with source numericalaperture NAs. As shown in FIG. 3 the effective source (ES) numericalaperture NAs lies within lens pupil (P) 304 which has numerical apertureNA.

FIG. 4 is a diagram illustrating an opaque disk 310 with a hole 312 init that can be used as one example of an illumination modifying optic IM214. In the example shown in FIG. 4, the hole 312 is in the center ofthe opaque disk 310. Positioning the opaque disk 310 at the CAS 216 ofthe PLT 220, see FIG. 2, reduces the numerical aperture NAs of theeffective source ES. Reducing the numerical aperture NAs of theeffective source ES increases the number of resolution elements that canbe imaged onto the lens pupil P 304. For example, a typical minimumsource NAs achievable in a modern scanner is about 0.06, and a typicallens pupil has a numerical aperture of NA of about 0.2. Thus, the numberof resolution elements, or NAs, that will fit within the entrance pupilwill be:Nr=NA/NAs=0.2/0.06˜3.33   (Eq. 1)

In the above, NA/NAs are measured on the reticle or entrance pupil sideof the imaging objective. Inserting the disk 310 alters the numericalaperture NAs of the effective source ES to a smaller value. For example,the NAs may be reduced to about 0.01, and in accordance with Equation 1,the number of resolution elements across the pupil is nowNr=0.2/0.01=20. Other NAs values may be used to produce a desired numberof resolution elements across the pupil.

FIG. 5 illustrates the numerical aperture S′ 330 of the effective sourceES after the opaque disk 310, being used as illumination modifying optic214, has been placed at the CAS 216. As discussed in FIG. 4, positioningthe opaque disk 310 at the CAS 216, see FIG. 2, reduces the numericalaperture NAs of the effective source ES (204). Thus, light emitted fromthe effective source ES and incident on lens (L) 206, see FIG. 2, has areduced numerical aperture in accordance with the size of the hole 312in the disk 310, that is being used in this example, as image modifyingoptic (IM) 214.

Returning to FIG. 2, ray bundles of light incident on the lens 206 arebent as they pass through the lens 206 so that chief rays with differentangles are incident at different points of the reticle encoded face (EF)224. The variety and extent of the angles of the chief rays aresufficiently diverse so as to overfill the pupil P of lens 206. As notedabove, the lens pupil is the image of the aperture stop 232 imaged as itwould be viewed from the light source 210. The lens 206 can be differentoptical elements, for example, a refractive lens, a conical lens,diffractive optic, or compound lens.

As shown in FIG. 2, a chief ray C1 and marginal rays RMA and RMB areshown incident on a measurement fiducial M1. Referring to FIG. 5, thechief ray C1 is the center of the entire ray bundle emanating from theeffective source (ES) 204 and incident on fiducial M1, while RMA and RMBare representative marginal rays, or rays that are on the edge of theeffective source (ES) 204 with numerical aperture NAs. Fiducial M1 islocated on the optical axis of the lens (L) 206 and thus the chief rayC1 emerges from the lens 206 perpendicular to the encoded face (EF) 224of the reticle 208. A different chief ray C2 is incident on themeasurement fiducial M2, and because it is not incident on the opticalaxis of lens 206 but is at a displacement h from the optical axis(h=distance between fiducials M1 and M2) it emerges from the encodedface (EF) of the reticle 208 at an angle q from perpendicular. The angleq, in a paraxial approximation, is equal to:q=h/f   (Eq. 2)where f is the focal length of the lens (L) 206. Referring to FIG. 5, inthe pupil (P) 304 the chief ray C2 that emerges from the encoded face(EF) 224 of the reticle 208 is centered on an effective source S″ region334 as a result of the lens bending action and the angular offset fromthe chief ray C1 that emerged perpendicular to the encoded face (EF) 224of the reticle 208 as approximated by Equation 2.

As shown in FIG. 2, an effect of the illumination matching optic (IMO)202 that includes an image modifying optic (IM) 214 is to create, atdifferent points on the encoded face EF 224, narrow ray bundles havingchief rays at angles that vary regularly as a function of position on EF224.

So the effect of the illumination matching optic IMO is to create atdifferent points on an encoded face (EF) a low partial coherenceillumination with narrow ray bundles whose chief rays vary regularly asa function of position on the EF. The IMO for this embodiment includesof a lens L, 206 attached to a reticle, 208, and the image matchingoptic IM placed at the conjugate aperture stop CAS, 216.

Encoded Face

As illustrated in FIG. 2, the measurement fiducial M1 on encoded face EF224 of the reticle 208 is typically at the object plane of theprojection lithography tool PLT 220. FIGS. 6A-6G show examples ofdifferent types of measurement fiducials that can be used for thefiducial M1 on the encoded face (EF) 224 of the reticle 208. In FIGS.6A-6G, dark areas represent regions of the encoded face 224 (see FIG. 2)that allow light to pass through. For example, the dark areas can beopenings in a chrome surface covering the encoded face (EF) 224 for adark field mask, which is the typical situation. However, the dark areascould represent regions of the encoded face 224 that do not allow lightto pass, thereby making a bright field mask. FIGS. 6A-6C areillustrations of typical measurement fiducial shapes of a square torus,a cross, and a box, respectively. These types of fiducials are oftenused as measurement fiducials for an overlay tool. FIGS. 6D-6Gillustrate examples of measurement fiducials that are commonly used aswafer alignment marks, which are marks read by a stepper or scanner forpurposes of aligning a wafer.

Generally, measurement fiducials should be compact so that across aprojected field point (FP′, 602) (discussed below in connection withFIG. 8) there are approximately Nr (the resolution across the exitpupil, see equation 1) features that can be distinctly measured. Inaddition, generally a fiducial needs its transverse positions configuredsuch that they can be accurately measured relative to one another. Aprojected field point (FP′) is a region of the PLT projection that hasmeasurement fiducials within it. Measurements made on the projectedfiducials within a field point are used to determine the aberration ofthe PLT at the field point (FP). If P is the spacing between measurementfiducials (the measurement fiducials are shown schematically as squaresin FIG. 7) on the encoded face, then a paraxial approximation of thenumber of projected measurement fiducials for a single field point willbe:Np=2*NA*f/P   (Eq. 3)

Setting the number of measurement fiducials for a single field point(Np) equal to the number of resolution elements (Nr) that will fitwithin the entrance pupil of the lens 206 associated with each fieldpoint allows determination of the ratio f/P. Therefore, by choosing thelens focal length, f, any sized field point can be accommodated throughthe choice of measurement fiducial.

A consideration in selecting the lens and measurement fiducial is thatgenerally it is desirable that the change in aberration over the fieldpoint FP be small compared to the size of the aberration. The fieldpoint is the collection of projected measurement fiducials arriving at asensing plane SP whose chief rays pass through a single lens 206. Asdiscussed below in connection with FIG. 11, there are generally aplurality of lenses 206 corresponding to a plurality of field pointswherein each field point includes a plurality of measurement fiducials.In modern scanners a field point typically corresponds to a region lessthan about 5-20 mm at the reticle. If isoplanaticity is not maintained(e.g. the change in aberration over the field point is significant, forexample greater than about 20%) a useful result for assessing themagnitude of the lens aberrations can still be achieved.

After passing through the illumination matching optic IMO 202 and themeasurement fiducial M1 (see FIG. 2) the intensity pattern at the pupilis altered by diffraction from M1. This alteration can be minimized byutilizing large geometries for M1, for example geometries that are muchlarger than λ/2*NA. The alterations can be further ameliorated byutilizing subresolution features and thereby producing an effectiveslope or gradient in the chrome transmission profile.

FIG. 10A shows an example of a measurement fiducial with a clear openingin chrome 802 on the encoded face EF 224 of reticle 208 that hasinterspersed sub resolution (<λ/2*NA) chrome openings 804 and spaces806. FIG. 10B illustrates examples of intensity profiles that can beproduced by measurement fiducials such as shown in FIG. 10A. In FIG. 10Bthe vertical axis 820 represents light intensity at the sensing, orimage, plane 242 and the horizontal axis 822 represents distance acrossthe projected measurement fiducial. The intensity profile of ameasurement fiducial that does not include a gradient would have asharp-shouldered intensity profile indicated by the dashed line 824. Theintensity profile of a measurement fiducial that includes a gradient,such as illustrated in FIG. 8A, has what is called a rounded intensityprofile 826. The rounding of the intensity profile reduces the lightintensity near the edge of the fiducial and thereby helps reduce theeffects of the diffraction. In either case, if the geometry and detailsof the measurement fiducials M1 are known, then the blurring, or effect,on the pupil intensity distribution S′ will be known and will becalculable by standard methods known to those skilled in the art.

Thus, the effect of the measurement fiducial M1 on the effective source204 provided by the IMO 202 is known and in general will not be so largeas to impact the number of resolution elements as calculated fromEquation 1. Furthermore, the size and placement pitch of the measurementfiducials and can be chosen by adjusting the focal length of the lens L206. The diffractive impact of measurement fiducials is readilyaccounted for according to conventional knowledge of those skilled inthe art.

Exposure

Referring again to FIG. 2, a sensing plane (SP) 242 is exposed resultingin an array of measurement fiducials that are projected onto the SP 242.The sensing plane (SP) 242 is sometimes also referred to as the imageplane or the wafer plane. FIG. 7 illustrates a 5×5 array of measurementfiducials that are projected across a field point (FP) 502. A fieldpoint (FP) is a region of the sensing plane containing the projectionsof measurement fiducials whose chief rays pass through the same lens (L)206 of a plurality of lenses (L) (not shown). In the example illustratedin FIG. 7, the measurement fiducials are uniformly spaced apart at apitch P. In other examples, the measurement fiducials may be spacednon-uniformly spaced apart at known pitches. The measurement fiducialsare projected through the pupil 304 of the lens (L) 206 as illustratedin FIG. 5, so not all of the measurement fiducials are projected ontothe SP 242 because some of the fiducials near the edge of the array offiducials are located outside the lens (L) 206 pupil (P) 304 and thuswill not pass through the aperture stop 232.

FIG. 8 illustrates the projections of measurement fiducials that passthrough the aperture stop 232 and are projected onto the SP 242 acrossFP′ 602. As shown in FIG. 8, the FIG. 7 measurement fiducials M3, M4,and M5 are not projected, or printed, onto the SP 242. In the exampleillustrated in FIGS. 7 and 8, measurement fiducials M3, M4, and M5 arenot printed or projected onto the SP because their chief rays and raybundles fall outside of the pupil (P) 304 of the lens (L) 206 asillustrated in FIG. 5. The measurement fiducials whose chief rays andray bundles pass through the pupil (P) are projected onto the SP 242.The measurement fiducials that are printed in this way, as shown in FIG.8, include M1′ and M2′.

FIG. 9 illustrates an example of a reference array (MO) 702 that isprojected onto SP using an ordinary reticle without the illuminationmatching optic IMO modifying the chief ray angles and effective sourcesize. The reference array (MO) 702 is impacted by aberrations in aconstant manner (over an isoplanatic patch) while projected measurementfiducials are impacted individually and differently by aberrations. Theresulting projected field point array (FP′) 602 can be used by overlaymetrology tools or other relative positional measurement tools to assessthe displacements in the projected measurement fiducials.

The sensing plane (SP) 242 (FIG. 2) is typically a photoresist coatedwafer at an arbitrary focal position of the IO 228. The effect ofarbitrary focal position is measured. The SP could also be an electronicsensor such as a CCD array with image matching optics, either part of orseparate from the projection lithography tool PLT 220.

Measurement

As noted above, FIGS. 6A-6C are typical measurement fiducials and can beused, for example, with a sensing plane (SP) that is a photoresistcoated wafer and a measuring apparatus, such as, an overlay metrologytool such as a KLA5200 or a scanner equipped with suitable patternrecognition (e.g. bar in bar pattern recognition). When accompanied bycomplementary alignment fiducials (schematically, reference array (MO)702 of FIG. 9) they can be read with high precision and repeatability(<3 nm). These features (when projected onto a wafer) could also bemeasured using an absolute metrology tool such as a Leica LMS IPRO. Inthis case, a reference array (MO) is not needed.

Wafer alignment marks, such as illustrated in FIGS. 6D-6G, can beprojected onto SP and used for either absolute or relative (overlay)metrology. To use in absolute metrology mode, the projected field pointFP′ without the reference array (MO), see FIG. 8, is exposed onto thesensing plane, typically a wafer. The projected image on the wafer isthen measured using the scanner wafer alignment subsystem to determinethe position of each projected measurement fiducial on the projectedimage. To use in an overlay or relative mode, the projected field pointFP′ with reference array (MO), see FIG. 9, is exposed onto the wafer.Thus, the MO array includes wafer alignment marks projected from thereference reticle that places the marks in proximity to the M1′ M2′ . .. or MA array. The wafer alignment system is then used to measureoffsets from the reference marks to the projected fiducials.

Another measurement technique utilizes an electronic detector (eitherintegral of, or detachable from the lithography tool) capable ofmeasuring the spacing between the projected fiducials either one at atime or in parallel. Such a system could utilize a reference array (MO)measurement.

Reconstruction

Ray bundles S′, S″ (FIG. 5) sample very different portions of the lens(L) 206 and are deviated at the sensor plane SP (FIG. 2) from theirideal positions (equal to position on encoded face/magnification) bylens aberrations in the manner described in Smith et al., U.S. Pat. No.5,828,455 (herein incorporated by reference). The projected deviationsdue to the lens aberrations is given by Equation 4:(dx,dy)=(λ/2πNAi)*∫d ² nW(n)∇(φ(n))/∫d ² nW(n)   (Eq 4)where:

A=wavelength of light from the effective source ES

NAi=numerical aperture on the image side=M*NA

N=transverse (nx,ny) direction cosine vector for position in exit pupil

W(n)=I(n)=intensity of effective source after passing through IMO=(forexample) intensity of S′ or S″ for measurement fiducials M1 and M2respectively

φ(n)=optical aberration of the projection lithographic tool.

For the projected measurement fiducial M1′, I(n) is >0 over the regionS′ of FIG. 5 so that the influence of the aberrations is just theweighted slope of grad (φ(n)) over this area of the pupil. Thus asdiscussed in the two Smith patents referenced above, taking positionalor differential offset measurements at an array of positions for theprojected measurement fiducials, we can reconstruct the imagingobjective (IO of FIG. 2) aberration. See also, “Method of ZernikeCoefficients Extraction for Optics Aberration Measurement” by T. Shiode,S. Okada, H. Takamori, H. Matsuda and S. Fujiware, SPIE Conference onMetrology, Inspection, and Process Control for Microlithography, March2002, incorporated by reference herein in its entirety.

Unit Device

FIGS. 11A-11C and 4 illustrate a device capable of carrying out aspectsof the invention. FIG. 11A shows in side view a unit reticle 902 withthree optical elements illustrated as lenses (L1, L2, L3) 904, 906, and908, respectively attached. Optical elements 904, 906, and 908 can be,for example, refractive lenses, conical lenses, diffractive optics,compound lenses or any combination of types of optical elements.

FIG. 11B shows in planar view, nine measurement fiducial arrays or fieldpoints (FP1, . . . FP9) 922-938. Each field point is associated with acorresponding lens (not shown). FIG. 11C illustrates an example of areticle 902 with a reference array (MO). As shown in FIG. 11C, thereticle includes a reference array (MO) with nine field points andcorresponding complementary fiducials associated with each field point.This reticle does not have lenses attached. Nine field points are shownfor purposes of illustration only, it should be understood that anydesired number of field points may be used in accordance with a desiredlens packing density and the desired transverse size at each fieldpoint. FIG. 4 shows the image modifying optic, IM1, that is insertedinto the effective source ES.

More Details and Variations

Equation 2 above describes the chief ray angle q as a function of imageheight. This is true only in the paraxial approximation. A more exactanalysis can account for the exact ray tracing through the lens (L). Inthe exact analysis, the ray angle is expressed by a formula:Sin(q)=a1*x+a2*x ² +a3*x ³+ . . .   (Eq. 5)Where the coefficients a1, a2, . . . are functions of the lens geometry.In practice, this formula is generally used.

Optical elements that can be used in place of a lens include conical oraxicon optics, compound (multiple) lenses, diffractive optics, andreflective optics. Especially in the case of reflective optics, theoptical element can be detached from the reticle that includes theencoded face and mounted in the projection imaging tool as a separatesubsystem. This may be desirable in X-ray or EUV systems utilizing allreflective optics.

The image modifying optic IN (see FIG. 2) can be a diffractive opticalelement. In that case, it will be located at a plane other than CAS andwill act to efficiently shape the source.

In another variation, the optical element can be replaced by a pinholeor small clear opening above the encoded face. Then rather thandecreasing the size of the effective source, an image modifying optic INthat increases the size of the sources S′ and S″ to overfill pupil isused. A diffuser is an example of such an IN. The optical element(pinhole) then restricts the ray bundle diameter incident and definesthe chief ray angle incident on each measurement fiducial on EF.

The foregoing description details certain embodiments of the invention.It will be appreciated, however, that no matter how detailed theforegoing appears, the invention may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive and the scope of the invention is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes, which come with the meaning and range ofequivalency of the claims, are to be embraced within their scope.

1. A reticle comprising: an array of field points, wherein each fieldpoint comprises an array of fiducials; and an array of optical elements,wherein an optical element is associated with each of the field points.2. A reticle as defined in claim 1, wherein the optical element is arefractive lens.
 3. A reticle as defined in claim 1, wherein the opticalelement is a conical lens.
 4. A reticle as defined in claim 1, whereinthe optical element is a diffractive optic.
 5. A reticle as defined inclaim 1, wherein the optical element is a compound lens.
 6. A reticle asdefined in claim 1, wherein the fiducials are scanner wafer alignmentmarks.
 7. A reticle as defined in claim 1, wherein the fiducials arestepper wafer alignment marks.
 8. A reticle as defined in claim 1,wherein the fiducials are square toruses.
 9. A reticle as defined inclaim 1, wherein the fiducials are crosses.
 10. A reticle as defined inclaim 1, wherein the fiducials include subresolution features to therebyproduce a gradient in transmission.
 11. A projection lithography toolcomprising: an effective light source comprising a light source; acondensing lens configured to accept light from the light source; and anillumination modifying optic between the light source and the condensinglens, wherein the illumination modifying optic is located at a conjugateaperture stop of an image plane of a projection lens system, theillumination modifying optic is configured such that light from thelight source that passes through the illumination modifying optic andcondensing lens forms a plurality of light ray bundles withcorresponding chief rays; a reticle upon which the light ray bundles areprojected, the reticle comprising: an array of field points, whereineach field point comprises an array of fiducials; and an array ofoptical elements, wherein an optical element is associated with each ofthe field points; and an upper lens group wherein the conjugate aperturestop is imaged by the combination of the condenser lens and the upperlens group onto an aperture stop, the upper lens group further includingoptics to image the fiducials onto a sensing plane.
 12. A projectionlithography tool as defined in claim 11, wherein the optical element isa refractive lens.
 13. A projection lithography tool as defined in claim11, wherein the optical element is a conical lens.
 14. A projectionlithography tool as defined in claim 11, wherein the optical element isa diffractive optic.
 15. A projection lithography tool as defined inclaim 11, wherein the optical element is a compound lens.
 16. Aprojection lithography tool as defined in claim 11, wherein themeasurement fiducials are scanner wafer alignment marks.
 17. Aprojection lithography tool as defined in claim 11, wherein themeasurement fiducials are stepper wafer alignment marks.
 18. Aprojection lithography tool as defined in claim 11, wherein themeasurement fiducials are square toruses.
 19. A projection lithographytool as defined in claim 11, wherein the measurement fiducials arecrosses.
 20. A projection lithography tool as defined in claim 11,wherein the measurement fiducials include subresolution features tothereby produce a gradient in transmission.
 21. A projection lithographytool as defined in claim 11, wherein the illumination modifying optic isan opaque disk with a hole in it.
 22. A projection lithography tool asdefined in claim 11, wherein the illumination modifying optic is adiffuser.
 23. An illumination source comprising: providing a lightsource; providing a condensing lens configured to accept light from thelight source; providing an illumination modifying optic between thelight source and the condensing lens, wherein the illumination modifyingoptic is located at a conjugate aperture stop of an image plane of aprojection lens system, wherein the illumination modifying optic isconfigured such that light from the light source that passes through theillumination modifying optic and condensing lens forms a plurality oflight ray bundles with corresponding chief rays; and placing a lens on aside of the condensing lens opposite from the illumination modifyingoptic, wherein angles of incidence of the chief rays within therespective light ray bundles onto the lens vary sufficiently to overfillthe lens pupil.
 24. The method of claim 23, wherein the illuminationmodifying optic comprises an opaque disk with a hole in it.
 25. Themethod of claim 23, wherein the illumination modifying optic is adiffuser.
 26. The method of claim 23, wherein the lens is a refractivelens.
 27. The method of claim 23, wherein the lens is a conical lens.28. The method of claim 23, wherein the lens is a diffractive optic. 29.The method of claim 23, wherein the lens is a compound lens.